What went wrong? When a user logged in on my deployed frontend ( https://domainA.app ), the credentials succeeded, but they were immediately treated as unauthenticated. Protected endpoints (like /stats/admin/* ) failed with a 401 Unauthorized status code. Crucially, this behavior did not happen on localhost. Why did it happen? This is caused by cross-site cookie restrictions enforced by modern web browsers. On localhost: The frontend ( http://localhost:5173 ...